1. Field of the Invention
The present invention relates to a hydrostatic vehicle driving system applicable to a working vehicle like that having a mower unit, namely, a lawn tractor, including a hydrostatic transmission preferably incorporated in a transaxle apparatus, wherein the speed reduction ratio exerted by the driving system is automatically changed in correspondence to the condition of load on an engine.
2. Related Art
Conventionally, it is well-known that a working vehicle like a lawn tractor having a mower unit is equipped with a belt type stepless transmission having a split pulley (hereinafter, “a belt transmission”) for avoiding overload on an engine of the vehicle. Generally, such a belt transmission is interposed between the output shaft of the engine and the input shaft of the transaxle apparatus incorporating a mechanical transmission like a multi-speed gear type transmission. The belt transmission automatically changes its speed reduction ratio according to the change of output rotary speed of the engine. In this regard, the speed reduction ratio of the belt transmission is smaller when the engine drives fast, and greater when the engine drives slowly.
This system is advantageous in that, while the engine drives fast, the smaller speed reduction ratio is established for efficient exertion of the capacity of the engine, and that, while the engine drives slow, the engine can be automatically prevented from overload because of the greater speed reduction. However, the tandem arrangement of the belt transmission and the transaxle apparatus interferes with minimization and cost-saving of the vehicle.
On the other hand, for being applied to a working vehicle such as a lawn tractor, there is a well-known conventional transaxle apparatus (an integral hydrostatic transmission, namely, an “IHT”) which incorporates a hydrostatic stepless transmission (hereinafter, an “HST”). The speed reduction ratio exerted by the HST, which corresponds to the ratio of capacity of a hydraulic motor to that of a hydraulic pump in the HST, is conventionally changed by a driver's manual operation of a movable swash plate of the variable capacity hydraulic pump. However, the conventional IHT used in such a working vehicle is not provided with a device automatically changing the speed reduction ratio of its HST in correspondence to output of or load on an engine.
Incidentally, even if the speed reduction ratio of such an HST in IHT for a working vehicle can be automatically changed correspondingly to load on an engine, there are still some cases such that the automatic changing of the speed reduction ratio is not desired. Particularly, if the working vehicle is a lawn tractor, it is preferable that, during its mowing work, the speed reduction ratio is not automatically changed, but manually changed only by a driver's will for preventing the lawn from being mowed unevenly.